1. Field of the Invention
The present invention provides a method for decreasing impedance of a power source in a printed circuit board, and more particularly, a method for decreasing impedance by depositing other ground planes in a routing layer of a power plane.
2. Description of the Prior Art
A printed circuit board, or PCB, is a support for elements of an electric device, and provides electrical connections between the elements. With highly developed electronic technologies, PCB density becomes higher and higher, so the PCB affects the anti-interference ability of the electric device a lot. Generally, even if a circuit diagram of the electric device is designed accurately, the PCB having some problems still decreases performance and reliability of the electric device. For example, regarding a motherboard of a computer, if the motherboard is not designed properly, signal exchanging between devices in the computer may suffer problems.
Considering the cost of production, a prior art motherboard usually has four routing layers. Please refer to FIG. 1, which illustrates a sectional diagram of a prior art motherboard 10. The motherboard 10 includes routing layers 12, 14, 16, 18, and dielectric layers 20, 22, 24. The routing layers 12, 14, 16, and 18 lay out a power plane, a ground plane, and circuit planes by means of copper plating and etching. The dielectric layers 20, 22, and 24 are made of a FR4 material for insulating the neighboring routing layers, where thicknesses of the dielectric layers 20, 22, and 24 are usually 4.4 mils, 48 mils, and 4.4 mils (1000 mils=1 inch). Therefore, if the power plane is laid out in the routing layer 12, and the ground plane is laid out in the routing layer 14, or the power plane is laid out in the routing layer 18, and the ground plane is laid out in the routing layer 16, there is a strong coupling between the power plane and the ground plane. The reason, as shown in FIG. 1, is that the distance between the routing layer 14 and the routing layer 16 is 48 mils, while the distances between the routing layer 12 and the routing layer 14, and between the routing layer 16 and the routing layer 18, are only 4.4 mils. However, when considering performance, production cost, and manufacturing, the circuit planes of the motherboard 10 are laid out in the routing layers 12 and 18, which are the outermost layers of the motherboard 10, and the power plane and the ground plane are laid out in the routing layers 14 and 16, meaning that the power plane and the ground plane of the motherboard 10 are separated by the thickest dielectric layer 22 made of the FR4 material. Regarding power supply, the ground plane is a return path of the power plane; that is, current outputted from the power plane flowing into the ground plane forms a loop. As a result, the closer the power plane and the ground plane, the smaller the loop inductance between the power plane and the ground plane, and as those skilled in the art recognize, the smaller the loop inductance, the lower the power impedance within a frequency range.
As to one effect of the thickness of the dielectric layer between the power plane and the ground plane upon the power impedance, please refer to FIG. 2 and FIG. 3. FIG. 2 illustrates a schematic diagram of a two-layer PCB 30. The PCB 30 includes a power plane 32, a ground plane 34, and a dielectric layer 30 made of the FR4 material between the power plane 32 and the ground plane 34. FIG. 3 illustrates a graph of input impedance to frequency when the thicknesses of the dielectric layer 36 are 5, 10, and 20 mils, and where the y-axis is input impedance, the x-axis is frequency, and lines 304, 302, 300 represent relations between input impedance and frequency when the dielectric layer 36 are 5, 10, and 20 mils respectively. As shown in FIG. 3, at a given frequency, the thicker the dielectric layer 36, the higher the input impedance of the PCB 30. Moreover, owing to a dominant pole of the PCB 30, the impedance of the PCB 30 rises quickly and suddenly in a range of high frequencies, or 1000 MHz to 5000 MHz, which limits the bandwidth of the PCB 30, causing a bottleneck of the PCB 30 with regard to high-frequency applications. That is, as the thickness of the dielectric layer between the power plane 32 and the ground plane 34 increases, input impedance of the PCB 30 increases, and the bandwidth of the PCB 30 decreases as well. Equivalent inductances of the dielectric layer 36, corresponding to 5, 10, and 20 mils thick at 100 MHz, are 2.85, 4.52, and 7.99 nH, so the thicker dielectric layer 36 causes larger equivalent inductance. Furthermore, in a range of low frequencies (lower than 1 GHz), the PCB 30 can be seen as a series connection of resistance and inductance. Because resistance varies little in the low-frequency range, inductance plays an important role, dominating the impedance. In short, the less the distance between the power plane 32 and the ground plane 34, the lower the equivalent inductance, the wider the bandwidth, and the smaller the input impedance.
As mentioned above, when considering performance, production cost, and manufacturing, a problem with prior art PCB designs is that the power plane and the ground plane of the prior art motherboard are separated by the thickest dielectric layer, which increases the input impedance of the power source, and decreases performance of the motherboard in the high-frequency range.